One of the last steps in the commercial production of molecular chlorine by the electrolysis of aqueous sodium chloride is usually liquefaction of the molecular chlorine. This is often accomplished by removing heat from the chlorine in a condenser cooled by a heat transfer fluid passing through a loop which includes the condenser for liquefying chlorine, one or more compressors, one or more heat exchangers for removing heat after compression, and an expansion valve, i.e., a single-loop system. Alternatively, a second heat transfer fluid passes through the a heat exchanger for cooling the first heat transfer fluid rather than for liquefying chlorine, and the cooled first heat transfer fluid is used in a separate loop to liquefy the chlorine in a condenser, i.e., a two-loop system.
Normally the chlorine and the heat transfer fluid used to liquefy the chlorine are substantially isolated from each other. A problem of particular concern in chlorine liquefaction plants, however, is one of safety in the event the chlorine and the heat transfer fluid used to liquefy the chlorine come into mutual contact. When such mutual contact does occur, it is usually, but not necessarily, due to failure of the condenser through which both materials flow. The failure can range from inconsequential, as for example where a very small leak and little or no reaction have occurred, to catastrophic, as for example where there is a major rupture of one or more tubes within the condenser and an explosion ensues. Between these extremes important adverse consequences can occur, as for example unacceptable contamination of the chlorine by heat transfer fluid or vice versa, with or without contamination by reaction products.
Heretofore trichlorofluoromethane, dichlorodifluoromethane, or chlorodifluoromethane has been used in a single loop system to liquefy molecular chlorine. Chlorodifluoromethane is reactive with molecular chlorine. Trichlorofluoromethane and dichlorodifluoromethane are not reactive with molecular chlorine at the temperatures used in chlorine liquefaction systems; they are, however, suspected ozone depleters and hence are being replaced by other materials. The replacements previously proposed contain more than a trivial amount of hydrogen in the molecule and are therefore reactive with molecular chlorine.
Trichlorofluoromethane or dichlorodifluoromethane has also been used as the second heat transfer fluid (i.e., the heat transfer fluid not passing through the condenser in which the chlorine is liquefied) in a two-loop system. In such cases the first heat transfer fluid has been reactive with molecular chlorine. In some instances the first heat transfer fluid is reactive because it contains enough hydrogen to support reaction; an example of such a heat transfer fluid is methylene chloride. In others, the first heat transfer fluid is reactive because it contains large amounts of water which can react with the molecular chlorine, albeit not explosively, as is the case, for example, with aqueous calcium chloride solution. The resulting acidic solution is very corrosive to many steels and other constructional materials.